The present invention relates generally to an integrated circuit module and more particularly is related to a laminated multi chip module having a ball grid array thereon.
For many years it has been customary to employ printed circuit boards ("PCBs") or printed circuit assemblies ("PCAs") as mediums for mechanically holding electronic components together and providing operative electrical interconnections between the components. The earliest PCBs were constructed of an insulating planar substrate (such as a glass fiber/resin combination) upon which a layer of conductive metal was deposited. The metal coating layer typically covered the entire surface of the substrate and was subsequently etched by a chemical process to form from the initial metal coating layer a predetermined pattern of conductive "traces" on the side surface of the substrate. Often, these electrically conductive traces were formed on both sides of the substrate to allow conductors to cross without coming into contact with one another. A plurality of mounting holes or "vias" were drilled through the metal layer(s) and the substrate, and were appropriately positioned to receive leads from the electronic components. This method of mounting electrical components on a circuit board is commonly referred to as "through-hole" mounting.
To complete assembly of a circuit board, the electronic components were placed on the PCB, either by hand or robotic machine, with the leads of the components passing through corresponding vias. Finally, solder connections were made to ensure reliable electrical contact between the components and the traces.
Initially, soldering was performed manually. Subsequently, more efficient machine-soldering techniques employing infrared ovens or solder baths were developed to speed manufacture of circuit boards and to ensure higher solder joint reliability. Under such machine-soldering techniques, the PCB and its components were heated while solder, under the influence of flux, was caused to contact the board and flow by capillary action into the vias, yielding a low resistance solder joint when cooled.
As circuit board technology developed, designers began to create circuit boards comprising many alternating substrate and conductive layer pairs, resulting in sandwiched circuit boards that could accommodate a higher component density. Such boards could accommodate ten or more conductive layers. Later, surface-mount technology allowed the leads to be soldered to solder pads on the surface of the circuit board, rather than requiring the leads to pass through vias to be soldered therein.
In addition to this circuit board construction evolution, the electronic components themselves underwent changes to accommodate higher density. First, discrete components were combined into integrated circuits ("ICs"). ICs were originally placed in dual in-line packages ("DIPs") each consisting of an elongated plastic body encapsulating the IC and a plurality of electrical leads coupled to the IC and arranged in a series extending from the two long edges of the body. The leads could either be through-hole soldered or surface-mounted. Unfortunately, the number of leads that a DIP could accommodate was a function of twice the length of the DIP body edges. Some improvement was made by providing packages having leads extending from all four edges of the body, but, even so, the number of leads was a function of the perimetral length of the body edges.
Next, in an effort to increase lead density further (to address, in particular, the increasing power and density of microprocessors and the stringent space requirements of notebook, subnotebook and personal digital assistant ("PDA") computers), designers developed quad flat packs ("QFPs") comprising a generally square body having leads extending downward from the lower surface of the body. The leads were typically arranged in multiple rows and columns, allowing the QFPs to accommodate more pins than DIPs. However, limitations in socket size and collective lead insertion force began to be problematical.
Currently, designers are focussing on ball grid array ("BGA") packaging wherein leads are dispensed with and replaced with a finely-pitched matrix of conductive contact surfaces on the lower surface of an otherwise conventional body. The circuit board to which a BGA package is to be mounted is conventionally provided with a matrix of corresponding surface mounted flat pad structures upon each of which is deposited a small quantity of solder. To mount the BGA package to the circuit board, the BGA package is temporarily clamped to the board and the board heated (typically by application of infrared energy), causing the solder to melt, fusing the corresponding surfaces together and yielding a strong mechanical and electrical connection when cooled.
In connecting a BGA electronic component package to the circuit board, the BGA package is typically placed on the appropriate side of the circuit board, using a high accuracy "pick and place" machine, in a manner such that the ball shaped solder portions of the BGA package contact the flat, surface mount pads which have solder paste screened onto them. The partially completed circuit board/BGA package structure is then subjected to an infrared solder reflow process to metallurgically and hence, electrically couple the surface pads to the ball shaped solder portions of the BGA package.
As is well known in the circuit board art, BGA packages that are soldered onto printed circuit boards using standard surface-mount technology are difficult to rework because when the BGA and corresponding pads are heated to a temperature sufficient to remove the multi chip module (MCM), the uniformity of the ball is typically destroyed in the reheating process. These limitations exist, of course, with single chip packages that have BGA mounting arrays on their undersides. Additionally, they are present, and even more undesirable, in more complex and expensive laminated multi chip module (MCM-L) packages having BGA mounting arrays built onto the undersides of their substrate portions. Currently, MCM-L's have BGA mounting arrays made of 63/37 Sn-Pb solder on the bottom surface which are then attached to the subsidiary package using the standard mount technology methods discussed above. These BGA arrays are either plated onto the surface of the MCM-L during the pattern plating operation at the bare board level or are screened on to the bottom surface metallurgy by a stencil and then infrared or vapor phase reflowed at the assembly level. While either of these methods provide for good solder ball formation, upon final assembly onto a planar, the device is extremely difficult to rework should some placement error cause package failure. The reworking difficulty arises from the fact that to disconnect the MCM-L component from the PCB, the solder ball and the solder pad to which it is connected must be reheated to melt the solder so that the component can be removed from the PCB. The melting of the solder usually destroys the critical uniformity of the solder ball and the mounting pad. Thus, the solder ball or the solder pad canner easily be reformed to provide for an effective connection when the PCB or component are reworked. Even if localized rework is utilized to remove the MCM-L, cleaning out the bottom surface metallurgy and redepositing solder to form solder balls is a time consuming and expensive process.
To over come this problem, some single chip carriers utilize expensive dielectric substrates that have a high transition temperature, e.g. from about 250.degree. C. to about 450.degree. C., and conventional IR reflow processes that allow the solder balls to be made of higher temperature solders, such as 10/90 Sn-Pb or 5/95 Sn-Pb solders. These solders have melting point temperatures that are less than the phase transition temperatures of the dielectric substrates to which they are being applied but that substantially exceed the melting point temperatures of the 63/73 Sn-Pb type solders that form the solder connector pads. Thus, when rework of the PCB board or the single chip carrier is desired, the board is heated to a temperature sufficient to melt the 63/63 Sn-Pb solder connector pad only, thereby allowing the chip to be removed from the board and leaving the higher temperature solder balls undamaged by the heating process.
The expensive dielectrics substrates that are typically used with these high temperature solders are made from materials, such as polytetrafluoroethylene or polyamides, which have phase transition temperatures capable of withstanding the higher application temperatures of the 10/90 Sn-Pb and 5/95 type solders. In contrast, however, these higher temperature solders cannot be utilized in low cost MCM-L's that use fiberglass resin-4 ("FR-4"), bis-maleamide triazine ("BT resin") or other conventional epoxy based dielectric substrate materials, which have phase transition temperatures of around 160.degree. C. to 190.degree. C. This is due to the fact that their phase transition temperatures are substantially less than the melting point temperatures of the higher temperature solders. As a result, attempting to form BGAs by infrared or vapor phase reflow using higher temperature solders could result in delamination and in some cases dielectric breakdown of the lower transition temperature dielectric substrates.
Therefore, as can readily be seen from the foregoing, it would be highly desirable to provide an improved apparatus and method for forming a solder ball interconnect mechanism comprising a high melting point temperature solder on a substrate such as a laminated multi chip module that has a phase transition temperature that is substantially lower than the melting point temperature of the solder used in the rest of the assembly in a manner eliminating or at least substantially reducing the above-mentioned problems. It is thus an object of the present invention to provide such improved connection apparatus and methods.